DC motors!!!

[maria]

Hi guys ,
There is a question related to commutation problems of DC motor. As we all know, the major problem is the “armature reaction” i.e. a magnetic field is produced by the current flowing in the armature windings, which distorts the field of the poles. Its most hazardous effect is called “flux weakening”. I.e. the overall flux under the entire pole faces decreases.
Q : I want to know, when the load on machine is increased, how flux decreases??? What happens to the speed of armature???

And in some DC shunt motors, due to this flux weakening effect “Run away condition” is achieved at which speed goes on increasing until it is disconnected from the supply or it is destroyed.
Q : How this rum away condition is achieved???
maria

 

[Phasor]

And in some DC shunt motors, due to this flux weakening effect “Run away condition” is achieved at which speed goes on increasing until it is disconnected from the supply or it is destroyed.

This is not a property of shunt motors, but actually of series motors. I'm a bit rusty with DC motors at the moment, so I can't say why this is, but I'll get back to you after I look it up in my texts...